Molded substrates are commonly used in a variety of different data storage media. For example, molded substrates can be used as optical data storage disks, or as part of holographic media or other data storage media. Optical data storage disks are widely accepted for use in storage, distribution and retrieval of large volumes of information. Optical data storage disks include, for example, audio CD (compact disc), CD-R (CD-recordable), CD-RW (CD-rewritable), CD-ROM (CD-read only memory), DVD (digital versatile disk or digital video disk) media, DVD-RAM (DVD-random access memory), and various types of rewritable media, such as magneto-optical (MO) disks and phase change optical disks.
Holographic data storage media, on the other hand, may provide higher storage density alternatives to conventional optical data storage disks. In a holographic medium, data can be stored throughout the volume of the medium rather than on the medium surface. Moreover, data can be superimposed within the same medium volume through any of a number of multiplexing methods. In this way, theoretical holographic storage densities can approach tens of terabits per cubic centimeter.
Optical data storage disk substrates can be produced by first making a master disk that has a surface pattern that represents encoded data on the master disk surface. The surface pattern, for instance, may be a collection of grooves that define master pits and master lands. The master disk is typically created by a relatively expensive mastering process.
After creating a suitable master, the master can then be used to make a stamper. The stamper has a surface pattern that is the inverse of the surface pattern encoded on the master. The stamper then can be used to mold large quantities of disk-shaped substrates in a mass production stamping process such as an injection molding process. Each substrate may contain the data and tracking information that was encoded on the master.
During a stamping process, an inverse of the surface pattern on the stamper becomes molded into each substrate, forming a collection of grooves or groove segments that define “pits” with respect to a plane referred to as the “land.” Typically, the stamped side of the substrate is then coated with a reflective layer, such as a thin layer of aluminum, and in the case of a CD, followed by a protective layer of lacquer. Data tracks on an optical disk substrate can be arranged in a spiral manner relative to the disk center, or alternatively, the data also can lie in a series of concentric tracks spaced radially from the disk center.
Some holographic media have a sandwiched construction in which a recording photopolymer layer is sandwiched between two molded substrates. The substrates can be created by an injection molding process similar to that used in the creation of optical data storage disks. For some for holographic media, the expensive mastering process may be avoided because surface patterns on the substrate may not be necessary. In other cases, however, holographic substrates may have surface patterns, for example, storing tracking information or the like. In those cases, the process of making a master can be used to define the tracking information on the molded holographic medium substrate.